Aluminum and its alloys have found a variety of industrial applications. However, because of the reactivity of aluminum and its alloys, and their tendency toward corrosion and environmental degradation, it is necessary to provide the exposed surfaces of these metals with an adequate corrosion-resistant and protective coating. Further, such coatings should resist abrasion so that the coatings remain intact during use, where the metal article may be subjected to repeated contact with other surfaces, particulate matter and the like. Where the appearance of articles fabricated is considered important, the protective coating applied thereto should additionally be uniform and decorative.
In order to provide an effective and permanent protective coating on aluminum and its alloys, such metals have been anodized in a variety of electrolyte solutions, such as sulfuric acid, oxalic acid and chromic acid, which produce an alumina coating on the substrate. While anodization of aluminum and its alloys is capable of forming a more effective coating than painting or enameling, the resulting coated metals have still not been entirely satisfactory for their intended uses. The coatings frequently lack one or more of the desired degree of flexibility, hardness, smoothness, durability, adherence, heat resistance, resistance to acid and alkali attack, corrosion resistance, and/or imperviousness required to meet the most demanding needs of industry.
It is known to anodize aluminum to deposit a coating of aluminum oxide, using a strongly acidic bath (pH<1). A drawback of this method is the nature of the anodized coating produced. The aluminum oxide coating is not as impervious to acid and alkali as other oxides, such as those of titanium and/or zirconium. So called, hard anodizing aluminum results in a harder coating of aluminum oxide, deposited by anodic coating at pH<1 and temperatures of less than 3° C., which generates an alpha phase alumina crystalline structure that still lacks sufficient resistance to corrosion and alkali attack.
Thus, there is still considerable need to develop alternative anodization processes for aluminum and its alloys which do not have any of the aforementioned shortcomings and yet still furnish corrosion-, heat- and abrasion-resistant protective coatings of high quality and pleasing appearance.
Aluminum and aluminum alloys are commonly used for automotive wheels since they are more corrosion resistant and lighter than traditional iron wheels. Despite the above-mentioned properties, bare aluminum substrates are not sufficiently resistant to corrosion; an aluminum oxide film tends to be formed on the surface and surface mars may readily develop into filiform corrosion. Conversion coating is a well-known method of providing aluminum and its alloys (along with many other metals) with a corrosion resistant coating layer. Traditional conversion coatings for aluminum wheels, namely chromate, are often environmentally objectionable, so that their use should be minimized for at least that reason. Non-chromate conversion coatings are relatively well known. For instance, conversion coating compositions and methods that do not require the use of chromium or phosphorus are taught in U.S. Pat. Nos. 5,356,490 and 5,281,282, both of which are assigned to the same assignee as this application.
Original equipment manufacturers for automobiles have specific corrosion resistance tests for their aluminum alloy wheels. While certain conversion coatings have been suitable for imparting corrosion resistance to many types of surfaces, they have not been deemed acceptable for imparting corrosion resistance to other surfaces requiring a relatively high level of corrosion resistance, such as aluminum alloy wheels.
Accordingly, is desirable to provide a coating, a composition, and a process therefor that are at least as reliable for the surfaces requiring a relatively high level of corrosion resistance as that provided by conventional chromate conversion coating. Still other concurrent and/or alternative advantages will be apparent from the description below.